Zirconium-titanium-tin alloys were produced by melting the pure elements in an arc furnace. The compositions of the alloys were selected to establish the equilibria and domains of the stable phases. Isothermal heat treatments at 1100 o C were performed for 750 and 1500 h in order to reach equilibrium among the phases. The characterization of the phases and the determination of their compositions at equilibrium were made by different techniques: metallographic observations, X-ray diffraction and electron microprobe analysis. Based on the obtained results, the isothermal section at 1100 o C on a triangular Gibbs phase diagram for the Zr-Ti-Sn system was drawn.